The present invention relates to a photographic wide-angle lens system of a retrofocus type. More particularly, the present invention relates to a wide-angle lens system that has an aperture ratio and a half-view angle on the order of 1:2.0 and 42.degree., respectively and which effects focusing by moving part (i.e. the rear lens group) of the system.
Photographic wide-angle lens systems are generally of a retrofocus type which has a sufficiently long back focus to permit installation on single-lens reflex cameras.
Wide-angle lenses of a retrofocus type that feature an aperture ratio and a half-view angle on the order of 1:2.0 and 42.degree., respectively are available in many versions as described in JP-A-No. 59-185307 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-B-No. 55-42364 (the term "JP-B" as used herein means an "examined Japanese patent publication") and JP-A-No. 55-164805. As auto-focus cameras have gained increasing acceptance these days, a need has arisen for the use of a lighter focusing section in order to reduce the load on the lens drive section. With wide-angle lenses of a retrofocus type, too, focusing by movement of only a part of the lens system has been desired since the front lens group of these wide-angle lenses is considerably heavy.
Typical examples of a wide-angle lens system that effects focusing by moving only part of it are described in JP-A-No. 62-249119 and JP-A-No. 62-291613.
The lens system proposed by JP-A-No. 62-249119 features a half-view angle of about 42.degree. and effects focusing by moving only the rear lens group of a retrofocus type system. The major problem with this system is that its aperture ratio of 1:2.8 does not insure brightness and that it experiences a marked change in the position of extra axial image plane during focusing.
JP A-No. 62-291613 propose various designs for effecting focusing by moving the rear lens group. In these systems, however, the rear lens group is divided into two units that are to move independently of each other and hence, the construction of the lens barrel becomes unduly intricate.
The present invention has been accomplished in order to solve the aforementioned problems of the prior art and its principal object is to provide an improved wide-angle lens system of a retrofocus type that features an aperture ratio and a half-view angle on the order of 1:2.0 and 42.degree., respectively. In this lens system, focusing is effected by moving only the rear lens group not only to reduce the load on the lens drive section but also to simplify the construction of the lens barrel. The system also insures high performance over a wide range from infinity to near distance.